Magnetic reconnection (MR) is a relaxation of the magnetic field's topology created by a local change in the magnetic field connectivity as described by [1]}. This process is due to relevant dissipative effects in localized regions, called current sheets, where the magnetic field lines are reconnecting. This phenomenon is continuously present in the solar chromosphere [2]}, [3]}, [4]}, [5]} as revealed by a wide range of observations, including the Interface Region Imaging Spectrograph [6]} and the Swedish 1-m Solar Telescope (SST) telescopes [7]}, [8]}, [9]} which provide unique observations of the dynamic outer solar atmosphere. In particular, recent observations with IRIS and SST [9]}, [11]} of strong brightenings in Si iv emission lines, “UV bursts”, reveal evidence for the formation of plasmoids in the presence of MR events. Observations with SST reveal the presence of blob-like features at Alfvénic speed in the Ca ii K line.
However, the comparison between spectral profiles of observed plasmoids and synthetic observations of reconnection suggests that 1- the number of plasmoids is large and 2- the observed motions within the reconnection site are much more complicated than those predicted by a classical single-fluid MHD approach, [12]}. UV bursts may span over extended regions from the upper photosphere, the chromosphere to the transition region (TR), thus, collisional and multi-thermal aspects of the plasma may play a role and need to be investigated [13]}.
